On bipartite distance-regular graphs with exactly two irreducible T-modules with endpoint 2

Abstract

Let $\Gamma$ denote a bipartite distance-regular graph with diameter $D \ge 4$ and valency $k \ge 3$. Let $X$ denote the vertex set of $\Gamma$, and let $A$ denote the adjacency matrix of $\Gamma$. For $x \in X$ let $T=T(x)$ denote the subalgebra of Mat$_X(\mathbb{C}$ generated by $A, E*_0, E*s_1, \ldots, E*_D$, where for $0 \le i \le D$, $E*_i$ represents the projection onto the $i$th subconstituent of $\Gamma$ with respect to $x$. We refer to $T$ as the {\em Terwilliger algebra} of $\Gamma$ with respect to $x$. An irreducible $T$-module $W$ is said to be {\em thin} whenever dim $E*_i W \le 1$ for $0 \le i \le D$. By the {\em endpoint} of $W$ we mean min$\{i | E*_iW \ne 0\}$. For $0 \le i \le D$, let $\Gamma_i(z)$ denote the set of vertices in $X$ that are distance $i$ from vertex $z$. Define a parameter $\Delta_2$ in terms of the intersection numbers by $\Delta_2 = (k-2)(c_3-1)-(c_2-1)p^2_{22}$. In this paper we prove the following are equivalent: (i) $\Delta_2>0$ and for $2 \le i \le D - 2$ there exist complex scalars $\alpha_i, \beta_i$ with the following property: for all $x, y, z \in X$ such that $\partial(x, y) = 2, \: \partial(x, z) = i, \: \partial(y, z) = i$ we have $ \alpha_i + \beta_i |\Gamma_1(x) \cap \Gamma_1(y) \cap \Gamma_{i-1}(z)| = |\Gamma_{i-1}(x) \cap \Gamma_{i-1}(y) \cap \Gamma_1(z)|;$ (ii) For all $x \in X$ there exist up to isomorphism exactly two irreducible modules for the Terwilliger algebra $T(x)$ with endpoint two, and these modules are thin.